Metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), and related alloys are known to be excellent catalysts. When incorporated in electrodes of an electrochemical device such as a fuel cell, these materials function as electrocatalysts since they accelerate electrochemical reactions at electrode surfaces yet are not themselves consumed by the overall reaction. Although noble metals have been shown to be some of the best electrocatalysts, their successful implementation in commercially available energy conversion devices is hindered by their high cost and scarcity in combination with other factors such as a susceptibility to carbon monoxide (CO) poisoning, poor stability under cyclic loading, and the relatively slow kinetics of the oxygen reduction reaction (ORR).
A variety of approaches has been employed in attempting to address these issues. One well-known approach involves increasing the overall surface area available for reaction by forming metal particles with nanometer-scale dimensions. Loading of more expensive noble metals such as Pt has been further reduced by forming nanoparticles from alloys comprised of Pt and a low-cost component. Still further improvements have been attained by forming core-shell nanoparticles in which a core particle is coated with a shell of a different material which functions as the electrocatalyst. The core is usually a low-cost material which is easily fabricated whereas the shell comprises a more catalytically active noble metal. An example is provided by U.S. Pat. No. 6,670,301 to Adzic, et al. which discloses a process for depositing a thin film of Pt on dispersed Ru nanoparticles supported by carbon (C) substrates. Another example is U.S. Pat. No. 7,691,780 to Adzic, et al. which discloses platinum- and platinum alloy-coated palladium and palladium alloy nanoparticles. Each of the aforementioned U.S. Patents is incorporated by reference in its entirety as if fully set forth in this specification.
One approach for synthesizing core-shell particles with reduced noble metal loading and enhanced activity levels involves the use of electrochemical routes which provide atomic-level control over the formation of uniform and conformal ultrathin coatings of the desired material on a large number of three-dimensional nanoparticles. One such method involves the initial deposition of an atomic monolayer of a metal such as copper (Cu) onto a plurality of nanoparticles by underpotential deposition (UPD). This is followed by galvanic displacement of the underlying Cu atoms by a more noble metal such as Pt as disclosed, for example, in U.S. Pat. No. 7,704,918 to Adzic, et al. Another method involves hydrogen adsorption-induced deposition of a monolayer of metal atoms on noble metal particles as described, for example, by U.S. Pat. No. 7,507,495 to Wang, et al. Each of the aforementioned U.S. Patents is incorporated by reference in its entirety as if fully set forth in this specification.
Although each of these approaches has been successful in providing catalysts with a higher catalytic activity and reduced noble metal loading, still further improvements in both the durability and mass-specific catalytic activity are needed for electrochemical energy conversion devices to become reliable and cost-effective alternatives to conventional fossil fuel-based devices.
One issue relating to the manufacture of conventional single nanoparticles or core-shell nanoparticles is the formation of MOH species on the surface of these nanoparticles that inhibit oxygen reduction reaction (ORR). A large number of low-coordination atoms on the surface of the nanoparticles is particularly prone to such oxidation and to gradual dissolution of the electrocatalyst over time. With continued operation, this tends to reduce the catalytic activity of the electrocatalyst and cause damage to the electrolyte membranes contained within a typical energy conversion device, thereby reducing its charge storage and energy conversion capabilities. One approach for synthesizing nanoparticles with a smaller number of low coordination sites and higher specific activity for the O2 reduction reaction is to synthesize nanowires and nanorods, which tend to have smoother surfaces. However, the conventional synthesis methods tend to use capping agents and surfactants to prevent agglomeration and facilitate growth of the desired shape and take hours or days to complete. An exemplary process is provided in Song et al. (Nano Letters, 2007, 7(12), 3650), which is incorporated herein by reference in its entirety. Unfortunately, removing these surfactants is quite difficult and inevitably causes increase of the particles' thickness, breaking of the nanowires, and an increase of the number of low coordination atoms.
There is therefore a continuing need to develop manufacturing methods that would avoid the use of capping agents and surfactants and can be completed within minutes, necessary for large-scale and cost-effective processes suitable for commercial production and incorporation in conventional energy production devices.